A technique has been conventionally known in which a target ahead of the own vehicle is detected with the use of a millimeter wave radar and in the case where the target thus detected satisfies a predetermined condition, a collision avoidance assistance process such as an alarm is executed. Among the targets, road structures such as manhole covers can be easily passed over by the own vehicle, and thus, the road structure does not require execution of the collision avoidance assistance process. According to a technique described in Patent Literature 1, a height of a target from a road surface is estimated from characteristics of a reflected wave of a millimeter wave radar, and based on the result of the estimation, a road structure and a target which requires the collision avoidance assistance process are distinguished.